In this section the properties and types of conjugated materials used in organic photovoltaics will be reviewed.
Conjugation arises from a series of alternating single and double bonds within a molecule. Semiconducting organic materials such as those used in organic pho- tovoltaics have carbon atoms that aresp2, or occasionallysp, hybridised [43]. This leaves theπ electrons available to delocalise into a cloud [44]. If the chain was uni- form this would lead to metallic behaviour. However, as single bonds are longer than double bonds a perturbation is produced in the electronic states of the mate- rial and a bandgap is formed in the electronic spectrum [13]. Ideally, this bandgap is similar to the bandgaps of inorganic semiconductors such as silicon and gallium arsenide, allowing the material to absorb visible light.
2.4.1
Conjugated Polymers
A polymer is a long chain of repeating subunit molecules, or monomers. The vast majority of polymers are those in which the carbon atoms which make up the chain are sp3 hybridised and hence form four σ bonds with neighbouring atoms.
Traditionally such polymers have been used in electronics as insulating and dielec- tric layers. In 1977 the first intrinsically conducting polymer, doped polyacetylene, was discovered [45]. Since 1990, undoped conducting polymers have emerged as potentially useful electronic materials, including in light emitting diodes and pho- tovoltaics. A range of conjugated polymers are shown in Figure 2.8.
Conjugated polymers have semiconductor-like electronic properties but the mechanical properties and processing advantages of polymers. In addition, the bandgap of the polymer can be varied with changes in the synthesis, enabling tailoring of the electronic properties. These properties are expected to lead to significant developments in solar cell technology [6, 13].
Figure 2.8: Molecular structures of a range of conjugated polymers. Electronic Properties
Unlike in inorganic semiconductors the charged species in organic semiconductors are not simply free electrons and holes. In polymers this is due to the quasi one- dimensional nature of the materials which leads to charges and excited states being accommodated by local changes in the chain geometry [43].
Polarons A polaron is a charge plus an associated lattice distortion. The physi- cal size of a polaron depends on the material; in poly(p-phenylene vinylene) (PPV) (structure shown in Figure 2.8) it has been estimated to be three to four monomer units [46]. Both positive and negative polarons are possible. Polarons have spin
1
2, and transport charge along the polymer chain.
A polaron increases the energy of the system and turns part of the molecule to a higher energy state. This is shown in Figure 2.9 for poly(para-phenylene) for the case of a positive polaron [47]. The charge induces a change in the bond
configuration leading to part of the chain changing from the aromatic structure to the higher energy quinoid structure.
Figure 2.9: The effect of a polaron on the structure of poly(para-phenylene) [47]. It is worth noting that the charge carriers in polymers are often referred to as electrons and holes instead of the more cumbersome but accurate negative and positive polarons.
Excitons When an electron is promoted from the HOMO into the LUMO it leaves a hole. These two charges are bound together to form an exciton. Alter- natively, an exciton can be thought of as a positive and a negative polaron bound by Coulomb interactions. Both singlet (where spins are anti-parallel) and triplet (spins parallel) excitons can exist, but for photovoltaic applications singlet excitons are of most interest. This is because photoexcitations produce singlet excitons due to conservation of spin [17].
Inter-molecular species The species discussed thus far have all been confined to one polymer chain, and hence are intra-chain species. As chains are in close proximity in a polymer film inter-chain species can also occur. These include excimers, inter-chain excitons, and aggregates.
An excimer is a complex between an excited state of one polymer chain and a ground state of another chain. In an inter-chain exciton the positive and negative charges are on different chains whilst an aggregate is essentially similar to an excimer except involves delocalisation over two or more chains in the excited and ground states.
Processing
The first developed conjugated polymers, such as PPV (Figure 2.8), are insoluble. There are several methods which can be used to overcome this problem.
The first solution is to synthesise materials which have solubilising side groups attached to the polymer backbone, thus allowing the material to be dissolved in common solvents. An example of such a polymer is MEH-PPV (Figure 2.8). These materials can be spun cast to form thin films.
The second method is to use a soluble precursor polymer [48]. Historically this method precedes solubilising groups. These precursor materials are non-conjugated and have a removable solubilising side group. As this material is solution process- able it can be spun cast into thin films for devices. Upon heating in a vacuum the side group leaves the polymer chain. This leaving group also removes a hydrogen atom from the carbon atom to which it was bonded. This results in a double bond linkage on the polymer chain, and the conjugation required. An example of this process is shown in Figure 2.10 for a xanthate precursor to PPV.
Figure 2.10: The thermal conversion of a precursor polymer. Under heat and vacuum the side group leaves the polymer, thus creating a conjugated polymer.
2.4.2
Conjugated Dendrimers
Another category of organic semiconductor used for optoelectronics is small mol- ecules (molecules with molecular mass less than a few thousand atomic mass units) [38]. One of the advantages of using small molecules is that they have greater chem- ical purity than polymers, with polymers more prone to defects due to their larger size. However, many small molecules cannot be processed from solution. Conju-
gated dendrimers generally have molecular masses lying between small molecules and polymers [11] and retain the chemical purity of small molecules but with the advantage of being solution processable.
One of the key differences between dendrimers and polymers is that dendrimers are highly branched. The conjugated dendrimer consists of three main parts: the core, the dendrons, and the surface groups (as shown in Figure 2.11). Conjugated dendrimers were originally designed to have efficient charge transport, specific luminescence, and solution processing for OLEDs [49]. The conjugation can be broken between the different groups (for example by using meta linkages [50]), and thus excitations are not delocalised across the entire molecule. The energy can be localised on the core of the dendrimer by using the correct choice of energy gradient between the the dendrons and the core [51].
Figure 2.11: Generic structure of a dendron molecule, adapted from [52]. The various components of the conjugated dendrimer control different proper- ties of the dendrimer. The core structure, which is either monomeric, polymeric or molecular, determines the absorption (or emission) of light; the dendrons con- trol charge transport and molecular spacing; and the surface groups determine the solubility of the molecule [49, 50]. The size of the dendrimer can be altered by adding more dendrons in a fractal fashion, as shown in Figure 2.12. The num- ber of such branchings is termed the generation of the dendrimer, and influences the charge mobility properties of thin films of the dendrimer [50]. Thus, the den-
drimer structure allows independent tuning of the absorption and the processing properties.
Figure 2.12: Structures of a family of dendrimers from generation (G) 0 to 3 [50]. As conjugated dendrimers are soluble, several materials with different cores (and hence different absorption properties) may be blended together and cast into a film. This allows absorption over a wider range of the solar spectrum than one material alone and potentially leads to enhanced performance in photovoltaic devices.